A sexual act that involves two participants. The person on the bottom stretches open his/her anus so that the person on the top may squat over said anus and excrete fecal matter directly into it. The bottom, with a bit of dexterity, can then proceed to push the "oil" back up the "pipeline" if he/she so desires.

"My Georgian girlfriend, Cynthia, and I were enjoying a delightful Caspian Pipeline, until Boris, from Russia, attacked Cynthia, claiming her as his own, and temporarily breaking the flow."